70 PRACTICAL PHOTO-MICROGRAPHY. 



object 0. The object-glass is also made to focus on O and 

 object-glass and condenser are working at their best when the 

 object, or the critical plane of the object, lies in focus of both 

 object-glass and condenser, and then only. The best resolution 

 of any plane of an object can only be achieved when the 

 object lies in the conjugate foci of objective, and condenser. 

 But the focus of the condenser is not usually long enough to 

 throw upon the object an image of the light large enough 

 to cover evenly the field of the objective and so with low 

 powers we have an image of the light only partially cover- 

 ing our field, and while scientifically speaking this is the 

 true critical image of our object, still, as a rule, a photo- 

 graph of an image so illuminated would be unsightly. There- 

 fore we make a compromise in one of several ways ; we 

 sacrifice to some extent the accuracy of our critical image in 

 order to make a more sightly photograph. And often we may 

 have so much resolving power " in hand," so to speak, that 

 we may sacrifice some of it without losing any of the necessary 

 resolution. Be it clearly understood that a critical image is 

 really the image of the radiant with the object intercepting 

 certain pencils of light ; this is fact for all cases of axial trans- 

 mitted light, but there are cases of oblique lighting and reflected 

 lighting where the object itself becomes the radiant. At pres- 

 ent we deal only with axial transmitted light. 



We effect the compromise mentioned above in various ways, 

 some better than others. The commonest way is to interpolate 

 between light and condenser a bull's-eye which collects pencils 

 of light from the radiant and transmits them parallel to each 

 other into the condenser. The result of this is that the rays 

 previously focused on a small area of the object are now spread 

 evenly over the whole field, and if the bull's-eye is properly used 

 in such a case there ought to be no falling off in the quality of 

 the image. It is important to keep the bull's-eye at a good 

 distance from the condenser, and the radiant must be at the 

 focal point of the bull's-eye. This is the usual method of 

 procedure with simple objects and low powers. The bull's- 

 eye is sometimes used alone as a condenser, being turned with 

 its convex side toward the radiant, and where an angle of not 



